Lapping - Lapping adds the final smooth finish on end—ground specimens and it serves as an alternative to end grinding in the lathe or the surface grinder. The simplest kind of lapping machine consists of a rotatin iron disc and a steel specimen holder which also moves carrying the specimen along the disc, when the disc starts to rotate. Any kind of oil-carried abrasives should be used and it is suggested that only water—borne abrasives should be used (HAWKES & MELLOR, 1970). To grind the specimen ends on the lapping machine, the specimen is placed in the steel specimen holder which is machined to accept a core (of a desired size) with a clearance of about 0.05 mu. The grinding compound is usually silicon carbide and aluminium oxide carried in water The lapping is normally done in two steps, i.e. , coarse lapping on a flat rotating iron disc using silicon carbide of about grade 100 first and then using silicon carbide of about grade 400 with aluminium oxide carried in water (Fig. 3), and fine lapping on a rotating grooved iron disc (Fig. 4). (4) Checking of Tolerances The specimen dimensions may be checked with a Vernier caliper or a micrometer during preparation. The squareness (perpendicularity of end faces to the specimen axis) may be checked using a steel tee-square. The flatness and smoothness of the specimen ends may be checked using a dial comparator (Figs. 5, 6 and 7). In the comparator shown in Figs. 5 and 6 the specimen is kept with its cylindrical walls touching the four nails protruding in the comparator and its bottom on the other nail coinciding with the center of the bottom end. The pate, which carries the dial gage, slides over the flat guide (Fig . 3) and by this tip of the dial gage is made to scan the whole upright face of the specimen. The maximum dial gage reading gives the deviation of the specimen end from smoothness. When the tip of the dial gage is made to scan the end surface along a diameter, the reading gives a measure of the deviation of end surface from being perpendicular to the specimen axis in the plane containing that diameter and the specimen axis. In the dial comparator shown in Fig. 7, the specimen is kept on a V-block with its axis horizontal and the principle of its operation is similar to the one described above. The allowable tolerances vary for each test. (5) Mounting of Strain Gages on Specimens If the deformational characteristics of a specimen are studied using electrical resistance strain gages, then they have to be bonded on the sunface of the specimen prior to testing to-allow the cementing medium to get hardened. Usually, few strain gages are attached both on axial and circumferential directions. Strain rosetts with two or more strain gages also can be used for this purpose. Specimen should be taken from the prescribed storage for each test and before certain surface requirements should be met in order to ensure a strong bond between the gage and surface. Preparation of the test surface prior to cementing the gage is normally the same for all gages and cements.